feat(railiance): implement CUST-WP-0032 Haskell build machine infra

Packer build definition, cloud-init autoinstall, GHCup toolchain script,
boot-time registration agent (state-hub + autossh dual tunnel), systemd
unit, key injection, remote-build Makefile, smoke test, and deployment
README. All 15 tasks complete.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

infra/build-machines/haskell/files/build-agent.service

@@ -0,0 +1,19 @@
+[Unit]
+Description=Haskell Build Agent — State Hub registration + SSH reverse tunnel
+Documentation=https://github.com/tegwick/the-custodian
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=simple
+User=build
+EnvironmentFile=/etc/build-agent.env
+ExecStart=/usr/local/bin/build-agent
+Restart=on-failure
+RestartSec=30
+StandardOutput=journal
+StandardError=journal
+SyslogIdentifier=build-agent
+
+[Install]
+WantedBy=multi-user.target